1. Field of the Invention
The present invention relates to optical attenuators used in optical communications systems and optical network technology, and particularly to an optical attenuator having high return loss.
2. Description of Related Art
Optical attenuators are widely used in optical transmission systems and optical networks. Optical attenuators are passive optical components that reduce optical power propagating in optical fibers, and may be categorized as either fixed or variable attenuators.
An important criterion for evaluating optical components such as optical attenuators is return loss. Many fiber transmission systems use laser diodes, and even a small amount of unwanted optical feedback can disturb the lasing characteristics of laser diodes. Such disturbances can include increased intensity of output noise, increased harmonic modulation distortion, and variation of linewidth. An optical attenuator having high return loss allows only minute optical feedback.
A conventional optical attenuator has a neutral density (ND) filter. The ND filter comprises an anti-reflection film stack or a haze film stack, either of which reduces optical feedback. However, such film stacks can only decrease optical feedback rather than completely eliminate it. Furthermore, phase tardiness caused by multireflected light between the attenuating device and the terminal of the input optical fiber still remains.
U.S. Pat. No. 5,109,468 discloses a fixed optical attenuator which is operational within a relatively wide spectral range. Optical fibers of the attenuator have refractive indices similar or equal to refractive indices of optical fibers of the optical transmittance system, therefore the attenuator has low Fresnel reflectance. The attenuator comprises an optical attenuating zone, in which optical signals can be attenuated. The attenuating zone is located in a light path, and comprises a couple of optical fibers which are poorly collimated with a predetermined tolerance. However, the attenuating zone requires exceedingly precise predetermined collimating tolerance. Therefore, in practice, the resulting control of Fresnel reflectance is limited, and the desired attenuation is difficult to obtain.
An object of the present invention is to provide an optical attenuator having high return loss.
Another object of the present invention is to provide an optical attenuator that is easily assembled and collimated.
To achieve the above objects, an optical attenuator in accordance with a preferred embodiment of the present invention comprises an input optical fiber and an attenuating device. The attenuating device is a neutral density (ND) filter. The filter has a refractive index matching that of the input optical fiber, to reduce Fresnel reflectance between the attenuating device and a terminal of the input optical fiber. One surface of the ND filter is not perpendicular to an incident light beam, thus preventing multireflectance between the attenuating device and the terminal of the input optical fiber. Therefore, return loss is increased.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: